Butch Malahide wrote at 2013-01-27 01:19:56
"Insidekick" by J. F. Bone is probably not what you're looking for, it's only a novelette and doesn't quite match the plot description, but it does feature a galactic agent who acquires a useful alien parasite while on a mission on a distant planet. There is an etext at Project Gutenberg:
Engineer who has read thousands of science fiction books and short stories. Can recommend stories on specific subjects and comment on technical feasibility. Especially good at older out-of-print books and authors.